Battery Questions...
 

I am looking into building a high mileage EV and I need to ask some questions. I used to work at Sears in the battery dept, so I have some experience with them, but very little in multiple setups.

1. In an EV with a 144V system, 12-12V batteries in a system in series, if all batteries are fully charged to 12.8V, and one is a higher AH capacity(11 batteries are 20Ah rated @100, and 1 is 20Ah rated at 150) Will the higher capacity be drawn off first? Or do all the batteries give off energy relatively equally? What about when you go to recharge? Will it recover correctly when you put a 144V charger on the system? How do you check?

I am assuming you have to check voltage with respect to specific gravity to verify, is this right or is there another way?

2.In an EV with a 144V system, could you run 10-12V batteries and 4-6V batteries if they were connected in series?

Thanks for your replies.

1) The batteries all discharge at relatively the same rate. If you drain 10Ah from the pack, you've drained 10Ah from every battery. The lowest Ah battery will be your weakest and limiting link. Same goes for charging.

Specific gravity is the best way of testing charge, but its just not practical for use while driving. Most guys use voltmeters and measure voltage under load.

2) Yes.

I know very little about EVs and distance, but I have 4 deep cell Sears die-hard batteries in my City-EL. They have held up since 2007 but I understand they were a product of one of the two large battery manufacturers. Sorry I can not be much help but those Sears batteries outlasted the City-EL!

you want to use batteries that are all the same size and age because as you draw them down you don't want one in the string to be at a higher voltage as it has to go thru the other batteries, if they were all parallel then it would not matter as much but if it was a string in series then it matters as it can have the same affect as hooking a charger up to the dead battery backwards.

Match your cells. Ideally, they should all be made at the same factory in the same batched, shipped on the same pallet to your door. Otherwise you're going to have lots of fun with unbalanced (different voltage) batteries. However, don't rule out your parallel battery pack idea. I still think that has merit.

~Jimbo